The present invention relates generally to a silicon nitride based sintered product and a method of producing the same making use of reactive sintering and, more particularly, to such a sintered product having fine or minute tissue and high mechanical strength.
As disclosed in Japanese Patent Application Laid Open No. 14635/1981, a method of producing a high-density silicon nitride based sintered product comprises: obtaining a calcinated product by adding to a Si (silicon) powder at least one powder of metal or oxides thereof selected out of Fe (iron), Co (cobolt), Ni (nickel), Mn (manganese), W (tungsten), Mo (molybdenum), Ti (titanium), Al (aluminum), Mg (magnesium) and Zr (zirconium); calcinating the product under an atmosphere of nitrogen at a temperature of 1300.degree. to 1500.degree. C.; dipping the calcinated product into a solution containing the metal, and again sintering the product while supplying a silicon-contained vapor at a temperature of 1350.degree. to 1550.degree. C.
When the molded product is being sintered by the aforementioned method, the added metal or the oxides thereof assume liquid or/and gaseous phase producing pores, inflations, cracks or the like within the final sintered product. Thus, the product is not suitable for high-temperature sintering, particularly, for thick-wall products.
In Japanese Patent Application Laid-Open No. 127266/1985, there is disclosed a method of producing a silicon nitride based sintered product which comprises preparing a molded product from a mixture of Si (silicon) powder and SrZrO.sub.3 (strontium zirconate) powder or powder of precursor thereof, nitriding the molded product, and thereafter heating it at a temperature of 1600.degree. C. or more. During the aforementioned method, the strontium zirconate becomes decomposed and reacts with Si.sub.3 N.sub.4 (silicon nitride) converted from silicon to produce also pores, inflations, cracks or the like within the final sintered product. Therefore, this method is not suitable for calcination of thick-wall products.
Another method of producing a silicon nitride based sintered product is disclosed in Japanese Patent Application Laid-Open No. 89462/1988 and includes the steps of preparing a raw material mixed powder by adding to and mixing with a silicon powder an oxide of at least one kind of element selected from a group consisting of Periodic Table, Group IIA, Group IIIA, Zr (zirconium) and Al (aluminum) and/or an oxide precursor; processing a molded product formed from the mixed powder at a temperature of from 1000.degree. to 1500.degree. C. under an atmosphere of nitrogen at 10 atmospheric pressure or more; and then further processing the molded product at a temperature of from 1600.degree. to 2200.degree. C. under an atmosphere of nitrogen at 1 atmospheric pressure or more.
A product disclosed in Japanese Patent Application Laid-Open No. 114169/1988 comprises a base portion of the ceramic reacted sintered material and a surface layer in which pores are impregnated with oxide sintering assistants or sintering aids. Also, a method of producing a silicon nitride based sintered product disclosed in Japanese Patent Application Laid-Open No. 52678/1989 includes the steps of molding a product from a mixed powder comprising essentially 40 to 80 wt. % of a silicon nitride powder, 20 to 45 wt. % of a silicon powder, and 0.5 to 15 wt. % of a compound of elements in Periodic Table Group IIIA; calcinating the molded product at a temperature of from 1150.degree. to 1400.degree. C. under an atmosphere of nitrogen pressed at 2 to 10 atmospheric pressure to form the silicon into nitride; and thereafter re-calcinating the product under an atmosphere of nitrogen at a temperature of from 1700.degree. to 2100.degree. C. to provide higher density.
Also know is a method of producing a silicon nitride product in which Si, Al.sub.2 O.sub.3 (alumina), and Y.sub.2 O.sub.3 (yttrium oxide) are used as main raw materials. An extremely fine amount of Fe (iron) is added to the main raw materials, which are reacted and sintered, after which calcination under low pressure is employed to produce a tissue of the silicon nitride having a degree of 96 wt. % of relative density. However, high strength is not provided by a silicon nitride based sintered product with a 96 wt. % of relative density.
Silicon nitride prepared by reacting and sintering a Si powder as in prior art has a grain dimension of the order of a few .mu.m. In addition, an expensive Si powder having a high purity of Si is present used. The resultant conventional silicon nitride based sintered materials do not exhibit high bending strength and high thermal expansion desired for products such as a piston pin of an internal combustion engine.
The object of this invention, therefore, is to provide an improved low cost silicon nitride product with high stength.